A. Field of the Invention
The invention relates generally to long-haul optical fiber transmission systems, more particularly, to long-haul systems including optical fiber with Raman amplification.
B. Background of the Invention
Signal degradation encountered when transmitting optical signals over long-haul optical fiber has greatly increased the need for improved optical signal amplification devices along the transmission path. Specifically, long-haul optical signal amplification presently suffers from amplification of noise along with the optical signal, resulting in a degraded signal to noise ratio (SNR) at the receiving node.
Presently, one method of long-haul signal amplification is achieved by utilizing a Raman amplification scheme. Raman amplification utilizes a pump laser optically coupled to the receiving node. The Raman pump laser provides an amplification signal propagating along the transmission path in a direction opposite the optical signal. As the amplification signal travels along the transmission path, energy is gradually transferred from the amplification signal to longer wavelengths of the optical signal through stimulated Raman scattering.
The power of the amplification signal is greatest near the output node of the long-haul optical transmission system where the pump laser inputs to the optical cable. Optical intensity of the amplification signal can be represented by the equation: Pintensity=(Laser Light Power/Aeff), where Aeff is the effective cross sectional area of the fiber.
Slope compensating optical fiber (SCF) is used in a section of optical fiber to compensate for the difference in dispersion at different wavelengths of the optical signals transmitted in single mode fiber (SMF). SCF also has a small Aeff when compared to other forms of optical fiber used for optical signal transmission, such as SMF. The small Aeff results in higher pump laser intensity which results in greater amplification of the transmitted optical signal.
Signal power going through transmission fiber is attenuated at the rate of about 0.1 dB/km to about 2 dB/km and, typically, of about 0.2 dB/km. Amplification signal power tends to degrade at an approximate rate of 0.25 db per 1 km of SCF optical fiber as it travels along the long-haul optical transmission system. Further, the minimum absolute dispersion of a particular wavelength of all of the wavelengths is typically in the range of 0 to 300 ps/nm. SCF optical fiber is utilized, at least partially, so that the difference between the absolute dispersion between the wavelengths is very small.
In Raman amplification, not only is the desired input signal amplified, but ambient noise introduced by a variety of sources as the input signal travels along a section of optical fiber is also amplified, resulting in a degraded SNR at the receiving node. The ambient noise being amplified is at least partially generated by multi-path interference (MPI) from double-Rayleigh back-scattering (DRBS) and Rayleigh back-scattering of amplified spontaneous emission (ASE).
The above mentioned noise degradation is particularly a problem in small Aeff fiber such as SCF fiber, primarily because most of the Raman gain occurs in the SCF section of the long-haul optical transmission system. The small Aeff of SCF fiber dramatically increases the fraction of Rayleigh back-scattering falling into its propagating mode. This leads to rapid growth of noise with increasing Raman gain. In addition at higher Raman gain, the total amplified signal power at the fiber section output becomes comparable to that of the pump and causes depletion. This, in turn, substantially degrades the Raman noise figure (NF).
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.
The invention relates to improvements in the amplification of the optical signal by substantially reducing or removing wavelengths propagating in a reverse direction.
In a first aspect, an apparatus for transporting an optical signal is provided comprising at least two sections of optical fiber, a directional wavelength selector positioned between the at least two sections of optical fiber wherein the directional wavelength selector selectively blocks wavelengths propagating in a reverse direction, and a pump light emitting device optically coupled to the optical fiber.
In a second aspect, a method of transporting an optical signal is provided comprising the steps of transporting an optical signal via an optical fiber comprising at least two sections of optical fiber, providing an amplification signal propagating in a reverse direction to amplify the optical signal using a pump light emitting device optically coupled to the optical fiber, and preventing pre-selected wavelengths from propagating in a reverse direction using a device positioned between the at least two sections of optical fiber.
In a third aspect, an optical transmission system including optical fiber, the optical transmission system transmitting in a predetermined wavelength range having a substantially central wavelength, is provided the system comprising at least two sections of optical fiber, a directional wavelength selector positioned between the at least two sections of optical fiber, and a pump light emitting device optically coupled to the optical fiber, wherein the directional wavelength selector prevents pre-selected wavelengths from propagating in a reverse direction.
In a fourth aspect, an optical transmission system including optical fiber, the optical transmission system transmitting optical signals in a predetermined wavelength range is provided comprising at least two sections of optical fiber wherein at least one section of the optical fiber comprises small effective area optical fiber, a pump light emitting device optically coupled to one of the sections of optical fiber wherein the pump light emitting device provides an amplification signal along the optical fiber, and a directional wavelength selector positioned between the sections of optical fiber wherein the directional wavelength selector allows the signal wavelengths to only propagate substantially in the forward direction and allows the amplification signal to propagate in the reverse direction.
In a fifth aspect, an apparatus for transporting an optical signal is provided comprising at least two sections of optical fiber, a pump light emitting device optically coupled to one of the sections of optical fiber wherein the pump light emitting device provides an amplification signal along the optical fiber, an optical circulator positioned between the sections of optical fiber comprising at least three ports Tx, Rx, and Lx, and a wave cancellation device positioned at the Lxport of the optical circulator. Signals entering port Rx circulate to port Lx, signals entering port Lx circulate to port Tx, and signals entering port Tx circulate to port Rx. The wave cancellation device reflects the amplification signal back into port Lx and substantially does not reflect other signals emerging from port Lx.
In a sixth aspect, a Raman optical signal amplifier is provided comprising an optical fiber comprising a first section of optical fiber and a second section of optical fiber, a directional wavelength selector positioned between the first and second sections of optical fiber wherein the directional wavelength selector allows signal wavelengths to propagate in a forward direction and selectively blocks wavelengths propagating in a reverse direction, and a Raman pump light emitting device optically coupled to the optical fiber for generating an amplification signal. The first section of optical fiber comprises optical fiber with an effective area less than 40 xcexcm2. The second section of optical fiber comprises optical fiber with an effective area greater than 50 xcexcm2.
Thus, a long-haul optical fiber transmission system consisting of sections of optical fiber with a directional wavelength selector has been described according to the present invention. Many modifications and variations may be made to the techniques and structures described and illustrated herein without departing from the spirit and scope of the invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the invention.